(1) Field of the Invention
The present invention relates to a semiconductor memory unit, such as a semiconductor disk unit, used as an external storage unit.
(2) Description of Related Art
In a high performance and high speed computer system such as have been proposed recently, the performance of a CPU (Central Processing Unit) is being improved rapidly with development of the semiconductor technology. Thus, it is required to improve the performance of an external storage unit coupled to the computer system. In compliance with the above requirement, a semiconductor disk using a semiconductor memory device capable of being electrically accessed at a high speed has been proposed as an alternative to a magnetic disk unit accessed mechanically. Due to the development of recent semiconductor technology, the capacity of the semiconductor memory device can become large and the price thereof can be decreased.
However, when the power supplied to the semiconductor device is cut off, data stored therein normally disappears (volatility). Thus, in the semiconductor memory unit used as an external storage unit, it is required to provide ways for making the data not disappear (non-volatility).
To achieve the above requirement, a magnetic disk unit is coupled to the semiconductor memory unit. When the power is cut off, the data stored in the semiconductor is transferred therefrom to the magnetic disk, and when the power is again supplied to the semiconductor memory unit, the data stored in the magnetic disk is returned therefrom to the semiconductor memory unit. However, as large amounts of data can be stored in the semiconductor memory unit, a long time is needed to transfer the data from the semiconductor memory unit to the magnetic disk unit and vice versa. Thus, it is difficult to save the data when a service interruption occurs; furthermore, a long time is required to restart the computer system to which the semiconductor memory unit and the magnetic disk unit are coupled.
Accordingly, in a conventional semiconductor memory unit, a back-up power supply is provided to the semiconductor memory device so that when the service interruption occurs, the data stored in the semiconductor memory device is maintained.
FIG.1 shows an example of a conventional semiconductor memory unit. The semiconductor memory unit shown in FIG.1 is a semiconductor disk unit.
Referring to FIG. 1, a semiconductor disk unit 2 is coupled to a host unit 1, and is used as an external storage unit. The semiconductor disk unit 2 includes a semiconductor memory portion 4 and an input/output controller 3. Data is stored in the semiconductor memory portion 4 in a format appropriate for a magnetic disk. The input/output controller 3 accesses the semiconductor memory portion 4 in accordance with instructions (e.g. a read/write command) supplied from the host unit 1 so that data read out from the semiconductor memory portion 4 is supplied to the host unit 1, and data output from the host unit 1 is written into the semiconductor memory portion 4. The semiconductor memory unit 2 is operatively coupled to a large-scale CPU in the host unit 1, and has a large capacity and high level functions. Thus, the semiconductor memory portion 4 and the input/output controller 3 require a large amount of electric power. Thus, an electric power is supplied to the semiconductor memory unit 2 from the same system power source (e.g. 200 v) as the host unit 1 which also requires a large amount of electric power. The semiconductor memory portion 4 is formed of a DRAM (Dynamic Random Access Memory), enabling high speed operations. However, data stored in the DRAM disappears when the service interruption occurs. Thus, batteries are used as the back-up power source for the DRAM (Japanese Patent Laid-Open Application No. 3-58111).
That is, the semiconductor memory unit 2 is provided with a main power supply circuit 5a for converting an AC voltage (200 v) supplied from the system power supply into a DC voltage (5 v), a secondary battery 6a, and a primary battery 6b. The semiconductor memory unit 2 is further provided with a power monitoring circuit 5b and a power selecting circuit 5c. The power monitoring circuit 5b constantly monitors whether or not the system power source is cut off and does so based on the output voltage of the main power supply circuit 5a. The DC voltage output from the main power supply circuit 5a is normally supplied to the semiconductor memory portion 4 via the power selecting circuit 5c. When the power monitoring circuit 5b detects that the system supply is cut off, the power selecting circuit 5c selects the secondary and primary batteries 6a and 6b so that the DC voltage is supplied from the secondary and primary batteries 6a and 6b to the semiconductor memory portion 4.
Even if the service interruption occurs so that system power source is cut off, the system power source is normally restored shortly therafter. In addition, a relatively small amount of dissipation power is required to maintain the data stored in the semiconductor memory portion 4 (the required DRAM). Thus, the capacity of each of the secondary and primary batteries 6a and 6b may be relatively small.
However, there are the following disadvantages in the above conventional semiconductor memory unit 2.
The system power source is generally used for the computer system. Thus, when the computer system is deactivated, the large capacity systems power source, which supplies a great amount of electric current, is cut off for safety reasons. That is, the computer system is managed in an operation style such that the computer system is turned off during nights and other long periods of time, such as holidays, by cutting off the system power source. In this operation style, the back-up power must be supplied from the batteries 6a and 6b to the semiconductor memory portion 4 during the time the computer system is stopped. In a case where the computer system is stopped during the weekend, for example, the batteries 6a and 6b must supply the electric power to the semiconductor memory portion 4 for at least 48 hours. In this case, large capacity back-up batteries 6a and 6b are required in the semiconductor memory unit 2, so that the semiconductor memory unit 2 becomes large and expensive.
In addition, a small-sized computer system, such as a workstation and a personal computer, may be stopped for an even longer time. In this case, the back-up batteries must have an even larger capacity.